Amyotrophic Lateral Sclerosis (ALS) and Parkinsonism dementia (PD)are neurological degenerative disorders that occur in three high incidence foci in the western Pacific: among the Chamorros of Guam and the northern Mariana islands, among the Japanese on the Kii peninsula and among the Auyu and Jakai peoples of West New Guinea. Clinically ALS and PD are two distinct disorders. However, preliminary analyses indicated that combining all three diagnoses (ALS, PD, and ALS+PD) into one affected diagnosis for genetic analyses, (thereby assuming any genetic effect on susceptibility to the two disorders was due to the same genetic mechanism) was reasonable. An Age, sex and birth cohort-specific liability was defined and segregation analysis was performed under both logistic and normal models for this liability at the time of disease onset. Under either model, purely environmental, Mendelian dominant and Mendelian recessive hypotheses could be rejected, but a two allele additive major locus hypothesis could not be rejected.